1. Field of the Invention
The present invention relates to electrical resistance elements and fluid heating apparatus utilizing a rigid, porous vitreous carbon body. In particular, the present invention relates to apparatus fitted with the porous vitreous carbon body which when heated by electrical or electromagnetic energy functions as a heater for fluids, particularly air, flowing through the pores in the body.
2. Prior Art
The use of vitreous carbon fibers woven into thin cross-section flexible mats as electrical resistance elements, particularly as relatively low temperature fluid heaters, is known. The mats are sometimes sealed in a flexible protective envelope. Such mats are not rigidly self-supporting and therefore require mechanical means to stay in place in a moving fluid stream and also have a relatively high resistance to fluid flow as measured by pressure-drop across the element. It would be valuable to have carbon resistance elements which are self-supporting, do not change in shape under flow conditions and have low pressure-drop characteristics. It would also be valuable to provide an efficient self-cooling resistor.
Nickel-chromium wire resistance heating elements are used almost universally in air stream heaters. Usually the elements are in the form of helical coils of the wire which are mounted on an insulator around which the air stream flows. There are two types of heaters, a low temperature heater which typically operates at an average wire temperature of about 380.degree. C. (716.degree. F.) and a high temperature heater which typically operates at an average wire temperature of 855.degree. C. (1571.degree. F.) as measured with an infrared thermometer. The heated wire in turn typically heats the air stream to about 135.degree. C. (275.degree. F.) for the low temperature heater or to 350.degree. C. (662.degree. F.) for the high temperature heater. The large wire temperature to heated air temperature differentials result because of the low effective heat transfer area of the resistance wire. At best, the response time of the heater at start-up is several seconds before the air is heated to the required temperature because of the thermal inertia of the wire. The high temperature of the wire at least with the high temperature heater means that the housing for the element must be designed to withstand melting from the heat radiated by the wire at these operating temperatures and the wire must be supported to prevent sagging from thermal softening with consequent touching of the housing at the elevated temperature, in addition to being supported in such fashion as to accommodate the large dimensional changes in the wire resulting from thermal expansion or element breakage. It would be very useful to provide a resistance element which had a very short response time at start-up and could be operated at low temperatures because of better heat transfer and yet was strong enough to resist distortion or damage by the flow of the air stream through the heated element and which was self-supporting and not subject to large dimensional changes with changes in temperature or to thermal softening with consequent undesired electrical contacts on element breakage.
Many carbon shapes when electrically heated in an air stream to a temperature which is nominally about 350.degree. C. (662.degree. F.) begin to oxidize significantly and then burn in the air, because the heat transfer of the shape to the air stream is irregular and locally portions of the carbon shape reach much higher temperatures causing combustion, which may be flameless but can be relatively rapid. Also, it is possible that minute breaks in the carbon shape form current breaks which cause localized overheating. For this reason carbon is not usually used in air heater applications where the operating temperatures are nominally about 350.degree. C. (662.degree. F.), where "nominally" means that there can be brief fluctuations to higher temperatures of up to about 600.degree. C. (1112.degree. F.). It would be a significant improvement if a carbon resistance heating element could be shaped so as to provide rapid heat transfer to the air to prevent localized overheating and current breaks and to allow operation of the element at surface temperatures very close to the desired output air temperature which is usually between 37.8.degree. C. to 204.4.degree. C. (100.degree. F. to 400.degree. F.) in most applications and can be as high as about 350.degree. C. (662.degree. F.) in some applications.
Also, there is a need for an economical apparatus and method where radiant electromagnetic energy, such as solar energy or microwaves can heat a fluid stream. A radiant energy absorptive element of the type described herein would be used for heat transfer.